Retroviruses are important agents in the etiology of cancers and other diseases. Human T-lymphotrophic virus type I (HTLV-I) causes adult T- cell leukemia (ATL). Based on ATL incidence data, early life exposure is critical for subsequent disease occurrence but new findings support adult exposure in some cases. Infective dermatitis of children is an HTLV-I associated syndrome with pre-leukemic potential. Spontaneous lymphocyte proliferation of peripheral blood cells from HTLV-positive individuals may contribute to leukemogenesis. American Indian populations may be the natural reservoir for HTLV-II infection, a virus previously associated with parenteral drug abuse on the basis of studies on native Americans in North, Central, and South America. Studies of human immunodeficiency virus type 1 (HIV-1) transmission have found that maternal-to-child transmission in twins often occurs in the perinatal period, possibly through exposure to products in the birth canal. Kaposi's sarcoma risk is markedly elevated in homosexual men, probably due to another infectious agent. Prophylactic therapies are undoubtedly prolonging life in severly immunosuppressed persons. With prolonged survival, an increasing proportion of HIV infected persons will develop lymphoma as their initial manifestation. The risk of non-Hodgkin's lymphoma in a large cohort of HIV-infected hemophiliacs was increased 200-fold but there was no excess of non-AIDS cancers. Other viruses associated with human malignancies are being studied. Hepatitus C virus may be linked to hepatocellular carcinoma. Human herpesvirus-6 (HHV-6), was identified in tissues obtained from patients with non-Hodgkin's lymphoma and Rosai- Dorfman disease. Epstein-Barr virus (EBV) was identified in the Reed- Sternberg cells of tumors obtained from patients with Hodgkin's disease. Chronic fatigue syndrome (CFS), a disease reportedly linked to cancer, was shown to be a heterogeneous group of illnesses, rendering its association with cancer questionable.